This invention relates to high-speed random access memory devices, and is more especially directed to an optical memory system in which light is employed to write and read data to and from memory cells of the systems. The invention is more particularly concerned with optical data storage systems in which the data can be written, stored, and read out non-destructively, or erased, as required, and which is capable of extremely rapid access, with process time on the order of picoseconds or faster.
A number of attempts have been made to produce a high-speed optical memory, i.e. one in which binary information is recorded and read out from a storage medium with a laser beam. For example, thin-film inorganic optical memories are discussed in U.S. Pat. No. 4,866,672, but these have drawbacks such as low crystalization speed, poor sensitivity, and inadequate reproduced-signal intensity.
Molecular-level data storage is described, e.g. in U.S. Pat. No. 4,032,901, in which a high-resolution scanning electron microscope is required to access the memory storage areas. The memory access time was rather high, on the order of ten nanoseconds to one microsecond.
U.S. Pat. No. 4,789,965 describes an optical data memory system that employs molecular pseudorotation, and discusses the possible use of photochromic materials in a memory device, where a color change would indicate the binary state. However, color gradients and variations in color intensity between "1" and "0" states have made it difficult to determine whether photochromic molecules were in one state or the other, and this ambiguity has severely limited practical applicability of such systems.
An optical switch for optically reading and writing data is described in U.S. Pat. No. 4,713,795. This system employs a nitroprusside medium, and also employs lasers of two different wavelengths, one for reading and the other for writing.
A frequency-selective optical memory is described in U.S. Pat. No. 3,896,420, which employs first and second optical oscillators, i.e., lasers, to write and read, respectively, using a crystal slab as a storage medium. These devices have not proved to be practicable for a number of reasons, among which is the need for extreme cryogenic conditions, i.e. operation at liquid helium temperature.
In recent years, attention has been focused on the potential use of light-transducing proteins to perform optical switching functions. These molecules, either in their native form, or as engineered compounds, obtained by genetic engineering or organic protein synthesis, have a number of attractive features including high speed and efficiency.
One such protein that has been studied is bacteriorhodopsin, which is the light-harvesting component of a halophilic microorganism Halobacterium halobium. This organism, which habitates salt marshes, grows this protein under conditions of oxygen deprivation, when the usual oxygen respiration route for synthesis of ATP from ADP becomes too inefficient to sustain growth. The bacteriorhodopsin molecule converts light energy into a hydrogen ion gradient that chemiosmotically drives the synthesis of ATP.
The photocycle of bacteriorhodopsin involves a ground state bR and a number of excited molecular states or photoproducts, K,M. At liquid nitrogen temperatures, e.g. 77 K, the bacteriorhodopsin molecule moves between its ground state bR and a primary photoproduct K, where red light (.lambda.=620 nm) brings the ground-state molecule bR to the photoproduct K, and green light (.lambda.=576 nm) operates to reverse this and returns the K photoproduct to the ground state bR. This is a high speed change of state, as the formation times associated with both the forward and reverse reactions are below five picoseconds.
At higher temperature (on the order of 200 K) there is photo-equilibrium coupling between the ground state bR and another excited mode M, where the formation times are moderately higher (on the order of 100 microsecond). In this case, the ground state molecule bR is photoreacted to the excited M state with green light (.lambda.=570 nm) in the order of picoseconds. The M state reacts to blue light (.lambda.=412 nm) to revert back to the bR state.
However, prior to this invention, no one has been able to employ bacteriorhodopsin in an optical switch, optical RAM, or other optical data storage device, and no one previously has appreciated the remarkable beneficial proprieties of long-term stability of the protein and its resistance to thermal and photochemical degradation; picosecond photochemical reaction times; high forward and reverse quantum yields that would permit use of low-light intensities for switching; wavelength-independent quantum yields; large shifts in the absorption spectrum characteristics that permit unambiguous and reproducible assignment of states; high two-photon cross-section for photoactivation which permits high storage density; and ability to form thin films with excellent optical properties.
No one in the art has previously appreciated the technological possibilities of an optical switch or memory device that employed bacteriorhodopsin or another photochromic medium, nor has anyone in the art proposed structure of a memory device that could successfully employ this protein or another photochromic molecule in a high speed random access memory device.